One important manifestation of cytopathology for many viruses is the inhibition of host cell macromolecular synthesis. We have recently shown that the matrix (M) protein of the prototype rhabdovirus, vesicular stomatitis virus (VSV), is capable of inhibiting the host-directed transcription of target genes in vivo in the absence of any other viral component. The goal of this project is to define the role of the M protein in virus-induced cytopathology and determine its mechanism of action. The M proteins of viruses isolated from persistent VSV infections will be characterized to determine the minimal M protein sequence required for inhibition and whether or not mutations in the M gene contribute to the reduced cytopathic effects of these viruses. Mutational analysis of the M protein will also be used to determine whether or not nuclear localization is required for inhibition of host gene expression. The relative sensitivity of different host genes to inhibition of their transcription by M protein will be determined and the factors that govern susceptibility to M protein-induced inhibition will be mapped. The mechanism of the M protein-induced inhibition of gene expression will be investigated using an in vitro transcription system to determine the step(s) that are inhibited by M protein These studies should provide new information on the mechanisms by which viruses cause pathological effects in their host cells and on the mechanisms by which viruses establish persistent infections in which their cytopathic effects are reduced. In addition, these studies have the potential to yield novel anti-viral strategies based on enhancement of cellular resistance to virus-induced damage to the host and reduction of virus replication.